Implantable pacing devices, such as, e.g., pacemakers, are used to treat a variety of cardiac conditions. Some pacemakers simply provide pacing pulses to a patient's heart at a fixed rate. More sophisticated devices contain sensing circuitry that allows the pacemaker to monitor a patient's heartbeat signals. For example, some pacemakers can monitor a patient's atrial heartbeat signals and provide corresponding ventricular pacing pulses, which allows the patient's cardiac output to be adjusted depending on the patient's intrinsic atrial heart rate.
Thus, certain therapy methods require that electrical activity in one area of the heart be sensed and analyzed while the same area or other areas of the heart are stimulated. For example, one area of the heart may be stimulated by high rate antitachycardia pacing (ATP) pulses. Unfortunately, these and other stimulation pulses can interfere with the sensing and analysis of the heart's electrical activity.
A similar problem exists in bradycardia pacemakers. For example, in dual chamber modes, the ventricular channel must be alert to sense R-waves while the atrial channel stimulates the atrium. In conventional pacemakers, this problem is typically addressed by blanking the sense amplifiers (e.g., disconnecting their inputs from the pacing/sensing electrodes) during a stimulation pulse and its associated fast recharge phase.
Blanking sense amplifiers may not be an adequate solution for sensing during ATP for several reasons. First, intervals between ATP pulses are typically very short (e.g., about 20 milliseconds during a 50 Hz burst). Therefore, the amount of time during which an amplifier is blanked for each pulse (e.g., about 12 milliseconds) is relatively long when compared to the time during which it is operational. This blanking interval may also be on the order of the duration of intracardiac activity or events that should be monitored (sensed and analyzed). Therefore, conventional blanking techniques tend to significantly compromise the ability to of the pacemaker to sense and analyze events of interest, especially during ATP therapy.
Secondly, certain cardioversion methods may require ATP pulses that have much higher amplitudes than pacing pulses used in conventional bradycardia therapy. These methods may also require that ATP pulse be delivered via high polarizing electrodes. Therefore, assuming an ATP pulse has a monophasic cathodal morphology similar to a conventional bradycardia pacing pulse, the polarization signal caused by the ATP pulse may be much greater than that caused by a conventional bradycardia pacing pulse. A large polarization signal can be disruptive to sensing from the stimulation electrode and possibly also from neighboring electrodes. One solution to this problem would be to lengthen the blanking interval and fast recharge interval. However, that would further compromise the ability of the pacemaker to effectively sense and analyze intracardiac signals during ATP therapy.
Thirdly, the monitoring of very low amplitude intracardiac signals that are produced during atrial or ventricular fibrillation requires that the pacemaker have a much higher amplification gain and/or lower sensing threshold setting than is typically required for detecting P-waves and R-waves in sinus rhythm. This tends to exacerbate both of the issues described above, because the more sensitive the amplifier the more prone it is to inadvertently sensing artifacts associated with a stimulation pulse, the fast recharge phase, and/or the blanking operation itself.
Consequently, there is a need for improved methods and arrangements that can be used to provide both cardiac stimulation pulses and effective intracardiac activity monitoring.